Amidst a generation which is so fond of reveling in the thought of an extension of all the limits of our knowledge, and is inclined to proclaim as true that which it wishes and hopes, investigators are not wholly wanting who very decidedly express their consciousness of these limits of our knowledge, and at the same time combine it with the most logical scientific reasoning and investigation. Even when in detail they reach these limits from the most varying points of view, and draw {149} them in different directions, they all agree in confirming the principle that it is one of the first and most indispensable conditions of successful investigation always to be conscious of the limits of its perception. Voices which remind mankind of these limits, are perhaps less popular, for man prefers to be reminded of the advances rather than of the limitations of his knowledge; but they are on that account the more worthy of our gratitude, for they keep us on the solid ground of the attainable from which alone sure progress in knowledge is possible. Among such philosophers we name Ulrici, and especially Lotze; among scientists, in the first place, two pioneers in their departments--namely, in the department of the mechanism of heat, Robert von Mayer--compare his "Bemerkungen ueber das mechanische Aequivalent der Waerme" ("Remarks on the Mechanical Equivalent of Heat"), and "Ueber nothwendige Consequenzen und Inconsequenzen der Waermemechanik" ("Necessary Consequences and Inconsequences of the Mechanism of Heat"), Stuttgart, Cotta;--and in the realm of the development of organisms, K. E. von Baer--compare his "Reden und kleinere Aufsaetze" ("Addresses and Essays"), 2 vols., St. Petersburg, 1864 and 1876. In this connection we have already mentioned the name of DuBois-Reymond. Otto Koestlin published two remarkable dissertations in this direction--"Ueber die Grenzen der Naturwissenschaft" ("Limits of Natural Science"), Tuebingen, Fues, 2d ed., 1874, and "Ueber natuerliche Entwicklung" ("Natural Development"), ib., 1875. In the latter he especially cautions against hastily confounding the laws of development of planets, development of the organic kingdom, and development {150} of the individual organisms. Recently, Wigand, in the second volume of his work already frequently mentioned, attempts, with an extreme energy which does too little justice to the representation and investigation of the still unsolved problems, to formulate the limits of the knowable.